Known in the past has been an exhaust purification system of an internal combustion engine which arranges an air-fuel ratio sensor or oxygen sensor at each of an upstream side and downstream side of an exhaust purification catalyst in a direction of flow of exhaust (for example, Japanese Patent Publication No. 2008-223644A and International Patent Publication No. 2014/118889A). In such an exhaust purification system, the output of the upstream side sensor is used as the basis for main feedback control of the fuel injection amount so that the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst matches the target air-fuel ratio. In addition, the output of the downstream side sensor is used as the basis for correction of the target air-fuel ratio in the main feedback control.
In addition, in the exhaust purification system which is described in the 2008-223644A application, at the time of engine cold start or otherwise when the temperature of the exhaust purification catalyst is low, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made to alternately change between the rich air-fuel ratio and lean air-fuel ratio every certain time interval. According to the 2008-223644A application, due to this, the exhaust purification catalyst can be quickly raised in temperature.